Gravure imaging system

ABSTRACT

A method of electrostatographic imaging utilizing a gravure member having a conductive backing with a uniform pattern of lands and valleys thereon, with the lands having a surface of a photoconductive material. A developing medium is applied to the valleys of the gravure member, leaving the land areas substantially clean. After application of the developing medium, the gravure member is uniformly charged and then exposed to a light and shadow image. The exposed member is then brought into transfer configuration with a transfer web.

United States Patent 191 Gundlach et al. Apr. 2, 1974 [54] GRAVUREIMAGING SYSTEM 3,559,570 2/1971 Martel 101/170 [75] In e ors: Robert w.Gundlach, victor; Joseph 3,561,358 2/1971 Weigl 101/170 Mammino; Alan B.Amidon, both of penfield; George can. Primary Exammer-Charles E. VanHorn Rochester, all of N.Y. I [73] Assignee: Xerox Corporation,Rochester, N.Y. [57] ABSTRACT [22] Filed; 27, 1971 A method ofelectrostatographic imaging utilizing a gravure member having aconductive backing with a PP N04 212,469 uniform pattern of lands andvalleys thereon, with the lands having a surface of a photoconductivematerial. 52 US. Cl. 96/1.4, 96/1 LY, 117/37 LE A developing medium isapplied the valleys of the [51] Int. Cl 603g 13/22 gravure memberleaving the areas Substantially 58 Field of Search 96/1, 1.3, 1.4;101/170; cleah- After application of the developing medium,

117/175 LE, 37'5 LY the gravure member is uniformly charged and thenexposed to a light and shadow image The exposed mem- [56] ReferencesCited bet is then brought into transfer configuration with a UNITEDSTATES PATENTS transfer 3,084,043 4/ 1963 Gundlach 96/1 11 Claims, 3Drawing Figures GRAVURE IMAGING SYSTEM This invention relates to anelectrostatographic imaging system, and more specifically, to anelectrostatographic imaging system using a photoconductive gravuremember. v

It is desirable to electrostatographically image using a gravure memberas the photoconductive member. Several attempts have been made to basereproduction systems upon a gravure photoconductor. For example, US.Pat. No. 3,561,358 to J. W. Weigl and US. Pat. No. 3,559,570 to R. W.Martel, et al., discuss methods embodying this concept. In both of thesepatents a photoconductive plate or cylinder is provided, the surface ofwhich is formed into a gravure pattern comprising uniformly spacedrecessed areas or tiny cells. In each, the surface of thephotoconductive gravure member is electrostatically charged and imagedin accordance with conventional xerographic tenchniques. In US. Pat. No.3,559,570 the photoconductive surface of the gravure roller, which isformed of a hydrophobic photoconductive composition, is contacted withan aqueous base developer. The unexposed charged areas of the plateaccept the ink while the exposed, discharged areas of the plate remainhydrophobic and repell the ink. The imaged or developed surface of theplate is then contacted with a copy sheet and the liquid developer istransferred thereto in an image pattern to produce a reproduction of theoriginal. In US. Pat. No. 3,561,358 the electrostatic latent image onthe gravure surface is developed in accordance with xerographicprocedures so as to selectively occlude the cells of the gravure member.The developer particles are then fixed into the cells. A printing ink isapplied to the resulting imaged member in such a manner that the inkfills the cells void of the developer particles. Upon contact of thesurface of the inked member with a copy sheet, a print of the desiredimage is realized.

While these two methods provide attractive altermates to methods usingdry developers, they suffer from the inherent disadvantage that thegravure member must be scrupulously cleaned after each pn'ntingsequence. This of course necessitates the use of complex and costlycleaning equipment to assure a clean copy upon each cycle free fromresidual ink from the previous cycle.

Accordingly, it is an object of this invention to overcome thisdisadvantage while retaining the major advantages of systems of thistype.

This and other objects are accomplished in accordance with the presentinvention which provides a method for electrostatographic imaging usinga gravure member comprising: uniformly charging the surface of a gravuremember having a uniform pattern of lands and valleys formed on aconductive support, wherein the valleys are filled with a developingmedium and the lands have a photoconductive surface which issubstantially free of developing medium; exposing the gravure member toa light and shadow image to cause development'of the image; andtransferring the image to a receiving web.

The invention will become more apparent from the accompanying drawingsand the ensuing discussion in which:

FIG. 1 represents a magnified cross-section through a gravure memberhaving the valleys filled with developing medium, as it is thought toappear immediately after application of an electrostatic charge thereto;

FIG. 2 illustrates the gravure member of FIG. I after exposure; and,

FIG. 3 represents a side-sectional view of an exemplary continuousimaging process according to the present invention.

Referring now to FIG. I there is seen a gravure member generallydesignated 1 comprising a conductive support base 2 having disposedthereon a raised pattern which is comprised of a uniform pattern oflands 4 and valleys 3. The land areas 4 have a photoconductive surface.

Any suitable material can be used to prepare the support base 2 for thegravure member of the present invention. Generally, the preferredsupport material should have an electrical resistance less than thephotoconductive layer so that it will act as a ground when thephotoconductive layer is electrostatically charged and exposed to light.Typical materials are aluminum; brass; steel; copper; nickel; zinc;conductive rubber; conductive glass, e.g., tin oxide coated glass; andmetalized plastic films such as aluminized polyethylene terephthalate orpolycarbonate films. The selection of the particular support basematerial used may depend upon the desired use of the gravure member. Forexample, if the master is to take the shape of a fiat printing platethen it may be more desirable to select a support substrate which willadd additional strength to the system. However, if the gravure member isto be prepared in the form of a roller or cylinder then it would begenerally more desirable to select a material which will provide thenecessary flexibility properties. When a rotogravure type of member isfabricated, the support surface can be in the nature of a hollowcylinder, or it can consist of a solid core such as a solid conductiverubber roller. The member can be rigid'or resilient.

The raised pattern on the surface of the support base can be formed byany suitable method so as to produce a spacing ranging from about 50 toabout 300 cells per inch. In order to obtain maximum resolution, it ispreferred that the number of cells per inch approach the upper limit ofthe specified operable range. The raised areas can be formed on theconductive support base and then coated with a photoconductive material,or a raised pattern of photoconductive material can be formed on arelatively smooth suppott base. It has been found to be desirable tohave at-least the surface of the land areas 4 coated with aphotoconductive material while the bottom of the recess 3 extendsthrough to the conductive support base. Because the difficulty informing a desired fine pattern of raised areas by applying aphotoconductive material in the desired raised pattern to a smotthsupport base, it has been found desirable to coat the land areas of asupport base having the desired raised pattern preformed thereon. Thephotoconductive layer may cover either the tips of the land areas along;or it can cover the tips of the land areas as well as the surfacesextending into the recessed areas. It is highly preferred that the tipsof the land areas alone be covered. A trigngular-helix patterned gravureapplicator roll has been found to be especially well suited to thepresent invention. However, other patterns having the desired spacing,uniformity and cell density are also acceptable. Trihelicoid-patternedrolls can be purchased from several commercial sources, or they can bemade by knurling or photofabrication techniques. Any suitablephotoconductive layer can be used as the charge carrier in conjunctionwith the process of the present invention. The photoconductive coatingcan comprise a photoconductor dispersed in an insulating bindercomposition, a solution of photoconductor and binder, or can consist ofa homogeneous photoconductive composition. The photoconductive layershould have a thickness of from about 6 to about I microns, with fromabout to about 60 microns being preferred. Typical of thephotoconductive compositions which are useful in the present inventionare those listed in U.S. Pat. No. 3,561,358.

The developing medium is applied to the gravure member by any suitablemeans which will substantially fill the recessed areas 3 while leavingthe land areas 4 substantially free of ink. Suitable inking techniquesare discussed more fully in copending, commonly assigned U.S.application Ser. No. 838,133 filed July 1, 1969 by G. P. Carr.Typically, this can be accomplished by uniformly coating the surface ofthe gravure member and cleaning the land areas 4 by means of a doctormember, e.g., doctor blades, non-skidding rollers, skid-rollers, and thelike. Skid-roll doctoring has been found to be an advantageous method ofmetering developing medium to rotogravure members. According to thistechnique a rotogravure member, which in this case has a photoconductivelayer over at least the tops of the land areas, and is illustrated by 6in FIG. 3, is rotated in contact with a resilient applicator roller 8which is passed through a developing medium source 10. Both therotogravure member 6 and the applicator roller 8 are rotated in the samedirection; however, the rotogravure member 6 is rotated at a greaterperipheral speed than applicator member 8. This causes application ofdeveloping medium to rotogravure member 6 by applicator roller 8 and asimultaneous cleaning of the land areas 4, shown exaggerated in relativesize on member 6, due to the skidding action of resilient roller 8 onthe surface of roller 6. Pressure between the two rolls is controlled bysuitable adjustment devices. The member 8 is rotated just fast enough tomaintain a bead of the developing medium. The skid roll 8 can be anyelastomer, for example, a fluorosilicone rubber.

After suitable application of developing medium, a uniform electrostaticcharge is applied to the surface of the gravure member in the dark. Thismay be done by corona discharge, and the applied charge can be eitherpositive or negative. The gravure member is then exposed to a light andshadow pattern image by any suitable means.

FIG. 1 illustrates a uniformly, positively charged gravure memberimmediately after charging. FIG. 2 illustrates a gravure member similarto that of FIG. I, wherein the member has been selectively exposed suchthat the left-hand area remains unexposed while the right-hand area hasbeen exposed. The member of FIG. 2 is shown as being ready for transferof the image to a receiving web which is preferably backed by conductivepressure member. If the conductive pressure member exhibits no potentialdifference relative to the discharged background areas, i.e., where itis grounded, a positive image is printed. A negative image can beproduced by applying a bias potential to the conductive pressure memberof like polarity and degree to that originally applied to the gravuremember. In FIG. 2, the charge on the developing medium has been fullydissipated by conduction to the conductive base 2, and the exposed areasof the photoconductive member have likewise been discharged.

Development is believed to occur by the combined effects of the electricfield on the surface tension of the developing medium and the directelectrostatic attraction of the liquid developing medium toward the landareas 4. However, this theory is set forth for aid in explanation onlyand should not be taken as limiting the invention. In general, thepreferred technique is to relax all fields in the exposed areas, whileallowing fields between the developing medium and the land areas tobuild in the unexposed areas. There are two altemative approachesavailable here: (1) use a thermofluid, e.g., wax-like developing medium,that is applied in the liquid state, solidified during charging andimaging, and softened to the liquid state by heating after the latentimage is completed by exposure; and (2) use a liquid developing mediumwhich remains liquid throughout the processing. In either case, thedeveloping medium should not be so conductive that it prevents propercharging of the photoconductive layer by conduction of the charge awayfrom the land areas 4. If a thermofluid developing medium is used, itcan be made to be relatively non-conductive in its nonflowable state,but relatively conductive after heating; or it can be made to lose itscharge over a relatively long period of time, compared to charging time,while in its nonflowable state. For a liquid developing medium the lightexposure level should be such that the rate of discharge of thephotoconductor in background (illuminated) areas, and of the developingmedium, should be mutually adjusted. Ideally, for sharpest developmentusing a liquid developing medium, the charge on the developing mediumshould become fully dissipated at the same time that the charge on theexposed areas of the photoconductive material becomes fully dissipated.Too rapid dissipation of the charge on the liquid developing mediumcould result in development of the entire gravure member beforeexposure. And, although subsequent exposure of selective areas wouldmitigate the effects of a not-too-severe premature development, it wouldbe difficult to impossible to achieve a final printed copy without ahigh background printing.

A suitable thermofluid developing medium may be made by adding dye orfinely dispersed pigment in a wax or other easily melted solid. Apreferred thermofluid developing medium can be made by adding 0.25 gm ofcrystal violet to 3.0 gm of Carbowax 1,500 polyethylene glycol and 6 gmCarbowax 6,000 polyethylene glycol (Carbowax is a product of UnionCarbide Corporation). The admixture is then diluted in 25 cc. of methylalcohol for coating purposes. After coating, the methanol is evaporatedbefore use for imaging. This composition becomes fluid on heating toabout C. If desired, nigrosine dye can be used in place of the crystalviolet to give a more permanent, black image, and beeswax or parafin waxcan be used in place of the Carbowax.

Suitable, liquid developing media can be those described in copending,commonly assigned U.S. application Ser. No. 839,801, filed on July 1,1969 by A. B. Amidon, et a1. Typically these may include one or moreliquid vehicles, colorants such as pigments and dyes, and dispersants.In addition, a variety of specialized agents may be employed forparticular functions. For example, viscosity controlling additives oradditives which contribute to fixing a pigment on copy paper may beemployed. These developing media will generally exhibit a bulkresistivity of between about to 10 ohm centimeter. As previouslydiscussed, the most important factor is to use a developing medium andlight exposure level such that the discharge of the developing mediumand the exposed areas are closely matched to minimize the fieldstherebetween. For example, a developing medium having a bulk resistivity(p) of 2 X 10 ohm-cm and a dielectric constant of 2 can be doctored intothe cups or grooves between the photoconductive ridges of a gravuremember. The discharge time constant (1) for the film of developingmedium is given by the formula:

'r=p X 8.85 X 10.

Therefore, the relaxation time for this ink is:

7: 2 10 x 2 x 8.85 x 10- 3.5 seconds Using in-place, full-framecharging, with voltages set to charge the self-developing photoreceptorplate in onehalf second, followed immediately by full frame exposureadjusted by lighting and lens aperture to give maximum electrostaticcontrast in 4 to 5 seconds, the discharge of the photoconductor in thebackground (lighted) areas closely coincides with that of the developingmedium itself. By operating in this manner, potential gradients betweenthe developing medium and the photoconductor are kept to negligiblelevels throughout the background areas, while the dark areas of thephotoconductor retain their charge and high potential, therebyelectrically attracting the adjacent developing medium. For slit chargeand exposure systems, the exposure time is again about 5 to 10 timeslonger than the charging time and should match the relaxation time forthe developing medium. Higher quality prints may be achieved by suchsystems by using a knife edge charging system, or, if rapid recycling isnot necessary, by using a photoconductor which exhibits persistentconductivity after exposure.

FIG. 3 represents a simple, exemplary apparatus for carrying out theimaging technique of the present invention. In this apparatus, there isseen a rotary gravure roller 6 having a trihelicoid patterned roller 12of a conductive material such as copper whereon the land areas arecoated with a photoconductive material such as polyvinylcarbazole and2,4,7-trinitro-9-fluorenone at a weight ratio of 4:1. The roller, whenin operation, is generaly rotated at a uniform velocity in the directionindicated by the arrow so that the surface of the roller passes inpressure contact with applicator roller or skid roller 8. Roller 8 isrotated in the direction shown by the arrow at a peripherial speed lessthan that of roller 6, such that developing medium is supplied to the recesses of the gravure member and the land areas are wiped clean. Roller8 can have a surface of fluorosilicone rubber, having a shore A-scalehardness of about 60, which is well bonded to a steel core to athickness of about one-eighth inch. The fluorosilicone roll surface isground smooth and polished to remove any imperfections. This material isdesirable primarily because of its inertness to many oils and solventscommonly used as developing medium vehicles or components, and itsresistance to compression set. Developing medium is supplied to the skidroll 8 by passing it through supply 10. The gravure roller 6, havingvalleys filled with and land areas substantially free of developingmedium, is uniformly charged in the dark by continuously rotating it atuniform velocity past charging unit 14, which can be a high voltagecorona discharge electrode adapted to supply ions or electric charges tothe surface of the roller. The charged gravure roller passes beneath ascanning image mechanism 16 or other means for exposing the chargedplate according to the desired image. The exposed roller surface thencontinues around so as to come into transfer configuration with copy web18 which is fed from supply roll 20 and passed up against the gravureroller surface by a grounded conductive transfer roller 22 which travelsat the same peripherial speed as the surface of the gravure roller. Thedesired image is thereby transferred to the copy web 18. Followingtransfer of the image to the copy web 18 the surface of the gravureroller may be immediately processed for the next cycle of printing ofthe same or a different image without requiring cleaning of the surfacethereof.

Where the transfer roller 22 is grounded and therefore has no chargerelative to the uncharged areas of gravure roller 6, positive,white-for-white and blackfor-black, copying of an original can beachieved. Where negative or reversal, that is, white-for-black andblack-for-white, printing is desired, a potential can be applied totransfer roller 22, equal in polarity and degree to the charge initiallyapplied to the surface of gravure roller 6. The initial charge appliedto the gravure roller can of course be either positive or negative.

To further explain the details of the present invention, the followingexamples are presented to illustrate but not to limit the particulars ofthe present invention. Parts and percentages are be weight unlessothewise indicated.

EXAMPLE 1 A copper letter press plate bearing a ISO-line pattern of topercent tonal half-tone dots raised 35 microns above the base plane iscoated with a solution of polyvinylcarbazole (PVK) and 2,4,7-trinitro-9-fluorenone (TNF) at a weight ratio of 4:1. The solution is prepared in a1:1 toluene-cyclohexanone solvent mixture which is used in sufficientamounts to provide a coating composition that is so viscous that it willnot completely coat the bottoms of the recesses in the halftone patternbut sufficiently fluid so that a uniform coating of the land areas canbe obtained. The plate is inverted during coating so that the coatingsolution will drain away from the bottom of the recesses to the landareas of the plate. The coating is metered by a number 30 wire-wound barand forced air dried at C. for about 15 minutes. The coating is cooledto room ambient conditions and electrically tested. The plate atsaturation potential is found to accept 300 volts either positive ornegative potential, and to discharge to about 20 volts residualpotential when exposed to white light (15 fcs). A developing mediumconsisting of about 45 percent light mineral oil, 27 percent alcolatedpolyvinylpyrrolidene and 28 percent of a resinated carbon black pigment,is applied to the photoconductive letter press plate. The excessdeveloping medium on the lands or top of the photoconductive dots issqueegeed away with a rubber wiper so that the ink is solely in thedepressions of the plate. In the dark the plate is corona charged usinga positive potential (V equals 280 volts) and exposed by projection toan image pattern (15 fcs). The image is then allowed to develop in thedark. The

developed image is then printed, still in the dark, by placing a sheetof Xerox 100 bond paper in face contact to the photoconductive plateandpassing a grounded conductive roll over the paper to achieve pressuretransfer. The paper or receiver sheet is removed from thephotoconductive plate and yields a positive image corresponding to theoriginal, that is white-for-white and black-for-black. The plate isreapplied with developing medium as described above, exposed, andallowed to develop as described above. Several images are obtained bythis procedure, reusing the same plate each time. In each case the plateis charged and exposed in the inked stage. Charging is possible becausethe developing medium is not so highly conductive as to divert thecorona current from the photoconductive lands (p is equivalent to about2 X ohm-cm.) Exposure is no problem because the developing medium doesnot cover the photoconductive areas.

EXAMPLE 2 Other images are obtained using the same procedure as inExample 1 above except that the photoconductive plate is charged using anegative potential. Here again, direct images are developed. Theseimages are equal in quality to those obtained when the plate is coronacharged positive as in Example 1.

EXAMPLE 3 Further images are obtained using the same procedure asdescribed above in Example 1 except that a bias potential of +300 voltsis applied to the conductive pressure transfer roll at the time of imagetransfer. The developing medium used has a resistivity of 2 X 10 ohm-cm.In this instance, reversal images, i.e., whitefor-black andblack-for-white of the original projected image are obtained.

EXAMPLE 4 Further images are obtained using the same procedure as thatof Example 2 except that a bias potential of -300 volts is applied tothe conductive pressure transfer roll at the time of image transfer.Here as in Example 3, reversal images are obtained.

EXAMPLE 5 Still further images are obtained using the same procedure asthat of Example 1 above except that a negative bias potential of 300volts is applied to the conductive pressure transfer roll at the time ofimage transfer. Here positive images with somewhat high background areobtained.

EXAMPLE 6 Yet further images are obtained using the same procedure asthat described above in Example 2 except that a positive bias potentialis applied to the pressure transfer roll at the time of image transfer.Here, as in Example 5, positive images with somewhat high background areobtained.

In an alternative embodiment of this invention, the photoconductivelayer can completely cover the surface of the gravure member. Thus, inthis embodiment, the recessed areas 3 and lands 4 would be made of orcoated with one continuous layer of photoconductive material such thatthe developing medium supplied to the recesses would be maintained outof contact with the conductive base 2. In this embodiment it is alsoimportant that the developing medium be opaque to the actinic energy forthe photoconductor. Operation according to this embodiment would be muchthe same as in the case where the recessed areas are not coated with aphotoconductor; however, here the images prepared in accordance with theprocedures outlined in Examples 1 and 2 would be reversal or negativeimages. This is so because the charge on the developing medium is notdissipated, and the exposed areas become developed due to the potentialdifference between the charged developing medium and the dischargedphotoconductor. Likewise, there will be no potential difference betweenthe charged photoconductor and the charged developing medium in theunexposed areas. The developing media used in this embodiment can bethose disclosed above, but here the upper limit on the bulk resistivityof the developing medium is not as critical. However, the developingmedium should have sufficient conductivity that its charge can bedissipated after each printing cycle by contact with a groundeddeveloping medium source or other suitable charge dissipation means.

It will be apparent to those skilled in the art that many modificationsand changes may be made without departing from the spirit of thisinvention which has as a principal feature the charging and exposing ofa gravure photoconductive member after application of a deformabledeveloping medium.

What is claimed is:

l. A method for electrostatographic imaging using a photoconductivegravure member comprising:

uniformly charging the surfce of a gravure member having a substantiallyuniform pattern of lands and valleys formed on a conductive support,said pattern having a cell spacing ranging from about 50 to about 300cells per inch, wherein the valleys are substantially filled with adeveloping medium and the lands have a photoconductive surface which issubstantially free of developing medium; exposing the gravure member toa light and shadow image to cause development of the image; andtransferring the image to a receiving web.

2. The method according to claim 1 wherein the gravure member has acylindrical surface with a trihelicoid pattern thereon.

3. The method according to claim 1 wherein the developing medium issupplied to the valleys by applying a layer of developing medium to theentire surface of the gravure member and then wiping the developingmedium from the land areas.

4. The method according to claim 3 wherein the developing medium isapplied by a skid roller.

5. The method according to claim 1 wherein the developing medium isapplied in the liquid state and remains liquid through the steps ofcharging, exposing, and transferring.

6. The method according to claim 5 wherein the developing medium has abulk resistivity of between about 10 to 10 ohm-centimeters.

7. The method according to claim 1 wherein the receiving web is backedby a grounded conductive member while in transfer configuration with thegravure member.

8. The method according to claim 1 wherein the receiving web is backedby a device which creates a charge equal in polarity and degree to thecharge inileys are substantially devoid of the photoconductive material.

11. The method according to claim 1 wherein the lands and valleys have asubstantially uniform coating of photoconductive material.

2. The method according to claim 1 wherein the gravure member has acylindrical surface with a trihelicoid pattern thereon.
 3. The methodaccording to claim 1 wherein the developing medium is supplied to thevalleys by applying a layer of developing medium to the entire surfaceof the gravure member and then wiping the developing medium from theland areas.
 4. The method according to claim 3 wherein the developingmedium is applied by a skid roller.
 5. The method according to claim 1wherein the developing mEdium is applied in the liquid state and remainsliquid through the steps of charging, exposing, and transferring.
 6. Themethod according to claim 5 wherein the developing medium has a bulkresistivity of between about 1010 to 1015 ohm-centimeters.
 7. The methodaccording to claim 1 wherein the receiving web is backed by a groundedconductive member while in transfer configuration with the gravuremember.
 8. The method according to claim 1 wherein the receiving web isbacked by a device which creates a charge equal in polarity and degreeto the charge initially applied to the surface of the gravure member. 9.The method according to claim 8 wherein the device backing the receivingweb is a conductive pressure roller.
 10. The method according to claim 1wherein the valleys are substantially devoid of the photoconductivematerial.
 11. The method according to claim 1 wherein the lands andvalleys have a substantially uniform coating of photoconductivematerial.